Phosphors are currently used in many important devices such as fluorescent lamps, lasers and crystal scintillators for radiation detectors, radiographic imaging and nuclear spectroscopy. An important property of any phosphor is its brightness, or quantum efficiency, which is the ratio of the number of photons emitted by the phosphor to the number of photons absorbed. Other important properties include the spectral region of maximum emission, optical absorption, emission decay time and density. Phosphors may be categorized as either intrinsic, where the luminescence is generated by the host material, or extrinsic, where impurities or dopants in the host material generate the luminescence.
In general, superior scintillators exhibit high quantum efficiency, good linearity of spectral emission with respect to incident energy, high density, fast decay time, minimal self-absorption, and high effective Z-number (the probability of photoelectric absorption is approximately proportional to Z5.) Specific scintillator applications determine the choice of phosphor. For example, scintillators used for active and passive radiation detection require high density and brightness, whereas scintillators used for radiographic imaging also require fast decay time.
Pending U.S. patent application Ser. No. 11/644,246, which shares a common assignee with the instant application, discloses novel nanophosphor composite scintillators utilizing a solid matrix or binder. The application notes that agglomeration of the nanophosphor particles may be caused by Van der Waals type or Coulomb type attraction between the particles, leading to non-uniform distribution of the nanophosphors. To prevent or minimize agglomeration, charge may be added or subtracted from the nanoparticle surface by adjusting the pH. Alternatively, surfactants may be added to the matrix to decrease agglomeration. However, neither of these approaches fully avoids agglomeration of the nanoparticles. Liquid scintillators are not disclosed.
Liquid scintillators are known in the art, but are limited by the quality of phosphor, and are subject to incomplete dispersion of the phosphor in the liquid. Conventional liquid scintillators are available, for example, from Saint-Gobain/Bicron and Eljen Technologies, Inc. However, none of the prior art liquid scintillators are loaded with high Z material suitable for applications such as gamma ray spectroscopy.
There is therefore a need for improved liquid scintillators and methods of producing liquid scintillators that prevents agglomeration at high volume loadings to produce uniform distributions of phosphor throughout the liquid.